Visible seams, commencing within the workpiece, should be provided with needle thread ends drawn towards the lower workpiece surface, wherein a needle thread forms the upper thread of the seam. This is no problem if the thread end of the needle thread is positioned in such a way that it is not clamped between the presser foot and the workpiece when the work presser is lowered onto the workpiece. In case that the latter nevertheless occurs, then the thread end will not be pulled through with the result that the seam becomes unsightly.
In order to assure the pulling through of the thread end through the workpiece thread pulling devices, so-called thread wipers have become known, at which a hook is movable in an angle with respect to the path of the needle and in the upper position of the latter. Thus, by the hook the thread end hanging down from the needle will be gripped at the descent movement of the needle and above the workpiece and then brought into a position above the presser foot, so that the thread end cannot be clampingly caught (DE-PS No. 25 26 694 corresponding to U.S. Patent Application Ser. No. 494,519). Such thread pulling devices are exposed to wear as they operate at a high operating frequency and almost without any lubrication. The manufacturing effort is considerable.
Moreover, blow devices are known appropriate to blow the thread end of the needle thread into a position, in which the thread end cannot be clamped between the presser foot and the workpiece.
These known devices are not applicable, if the workpiece to be processed is very thick. This applies for example to thick leather, heavy tent materials, heavy upholstery materials, upholstery mats and similar materials. In this case no sufficient clearance is available between the needle and the presser foot, so that the pulling devices cannot be used for space reasons, whereas blowing devices do not achieve an action. In the case that a thread pulling device is not applicable for space reasons, the operator must especially take care of the end of the needle thread while putting in a workpiece to be sewn, so that the end of the needle thread will not get clamped at lowering of the presser foot, respectively the presser feet, which, on the other hand, is time consuming. From German patent specification No. 276 162 it is known to prevent that the end of the needle thread released by a thread cutting operation will be pulled out of the needle due to the descending thread take-up lever. For this purpose the needle thread will be so clamped between the thread take-up lever and the eye of the needle by means of a special clamp and about simultaneously with the thread cutting operation that the thread take-up lever can only pull additional thread off the spool at its upward movement, without, however, pulling the free thread end out of the eye of the needle. In order to be no longer restricted to employ double-grooved needles, it is proposed in the German patent specification No. 276,162 to open the needle thread clamp temporarily during the penetration of the needle for the formation of the initial stitch of a stitch group following a thread cutting operation. After the looptaker has entered the needle thread loop, the needle thread clamp will be actuated again.
From German Gebrauchsmuster No. 66 03 307 a controlled thread clamp is known for the needle thread of a stitch group sewing machine, which is brought to a standstill automatically at the end of a work cycle, wherein the work clamp will be lifted, the threads will be cut off and the needle bar will be stopped shortly after its upper dead center. With this controlled thread clamp arranged on the needle bar only the threading-off of the needle thread out of the eye of the needle can be prevented by clamping the needle thread.
Moreover, it has become known from German patent specification No. 28 15 297 to modify this thread clamp, so that also an optimal length control of the thread end hanging out of the eye of the needle prior to sewing of a seam is rendered possible.
Also from German patent specification No. 1 141 165 a sewing machine with a thread clamp for clamping the thread end during the initial stitches is known, by which a threading-off of the thread out of the needle should be prevented.